source: CIVL/examples/mpi-pthread/mpi-pthreads-pie-collective.c@ a7f4a7a

/**********************************************************************
                        ProMCore 2008 
                    February 05 - 09, 2008


 Example 1.2      : mpi-Pthreads-pie-collective.c
    
 Objective        : Write an MPI-Pthreads program to compute the value
                    of PI by numerical integration of a function f(x)
                    =4/(1+x*x) between the limits 0 and 1.Pthreads and 
                    MPI Collective communication library calls are used.

                    This example demonstrates the use of

                    pthread_create()
                    pthread_join()
                    pthread_mutex_lock()
                    pthread_mutex_unlock()
 
                    MPI_Init 
                    MPI_Comm_rank 
                    MPI_Comm_size 
                    MPI_Bcast 
                    MPI_Reduce 
                    MPI_Finalize

 Input             : Number of Intervals.

 Output            : Calculated Value of PI.

 Created           : January 2008                                              
                     National PARAM Supercomputing Facility,C-DAC,Pune.        

 
 E-mail            : betatest@cdac.in                                          

************************************************************************/

#include <stdio.h>
#include <math.h>
#include "mpi.h"
#include <pthread.h>

double  intervalWidth, mypi = 0.0;
int     *MyIntervals;
int     MyRank, Numprocs, Root = 0, MyCount = 0;
pthread_mutex_t mypi_mutex = PTHREAD_MUTEX_INITIALIZER;

void * MyPartOfCalc(int myID)
{
        int    myInterval;
        double  myIntervalMidPoint;
        double  myArea = 0.0, result;

        myIntervalMidPoint = ((double) MyIntervals[myID] - 0.5) * intervalWidth;
        myArea += (4.0 / (1.0 + myIntervalMidPoint * myIntervalMidPoint));

        result = myArea * intervalWidth;

        /* Lock the mutex controlling the access to area. */

        pthread_mutex_lock(&mypi_mutex);
        mypi += result;
        pthread_mutex_unlock(&mypi_mutex);
}

double func(double x)
{
        return (4.0 / (1.0 + x * x));
}

int main(int argc, char *argv[])
{
        int             NoInterval, interval, i;
        double          pi, sum, h, x;
        double          PI25DT = 3.141592653589793238462643;
        pthread_t      *threads;
#pragma CIVL $assume 0 < NoInterval && NoInterval < 3;

        /* ....MPI initialisation.... */
        MPI_Init(&argc, &argv);
        MPI_Comm_size(MPI_COMM_WORLD, &Numprocs);
        MPI_Comm_rank(MPI_COMM_WORLD, &MyRank);

        if (MyRank == Root) {
                printf("\nEnter the number of intervals : ");
                scanf("%d", &NoInterval);
        }
        /* ....Broadcast the number of subintervals to each processor.... */
        MPI_Bcast(&NoInterval, 1, MPI_INT, 0, MPI_COMM_WORLD);

        if (NoInterval <= 0) {
                if (MyRank == Root)
                        printf("Invalid Value for Number of Intervals .....\n");
                MPI_Finalize();
                exit(-1);
        }
        h = 1.0 / (double) NoInterval;
        intervalWidth = h;

        /* Start Calcualtions to determine the number of intervals for each */
        /* processes of the Communicator. */

        MyCount = 0;
        for (interval = MyRank + 1; interval <= NoInterval; interval += Numprocs)
                MyCount++;

        MyIntervals = (int *) malloc(sizeof(int) * MyCount);

        for (i = 0, interval = MyRank + 1; interval <= NoInterval; i++, interval += Numprocs)
                MyIntervals[i] = interval;

        /* Start creating threads. */

        threads = (pthread_t *) malloc(sizeof(pthread_t) * MyCount);

        for (interval = 0; interval < MyCount; interval++)
                pthread_create(&threads[interval], NULL, (void *(*) (void *)) MyPartOfCalc, (void *) interval);

        for (interval = 0; interval < MyCount; interval++)
                pthread_join(threads[interval], NULL);

        /* ....Collect the areas calculated in P0.... */
        MPI_Reduce(&mypi, &pi, 1, MPI_DOUBLE, MPI_SUM, Root, MPI_COMM_WORLD);

        if (MyRank == Root) {
                printf("pi is approximately %.16f, Error is %.16f\n",
                       pi, fabs(pi - PI25DT));
        }
        MPI_Finalize();

}